The present invention relates generally to magnetic flowmeters. More specifically, the present invention relates to a magnetic flowmeter which automatically selects the current level for use with different flowtubes.
A magnetic flowmeter measures the volumetric flow rate of a conductive fluid by detecting the velocity of the fluid passing through a magnetic field. Magnetic flowmeter systems typically include a flowtube assembly and a transmitter assembly. The flowtube assembly is installed in a process piping line, either vertically or horizontally, and includes a pipe section, a coil section and electrodes. The coils are located on opposite sides of a cross section of the pipe. The coils, energized by a coil drive current from the transmitter, develop a magnetic field along the cross section of the pipe. Two electrodes are located across the pipe from each other along a line which is perpendicular to the magnetic field. Fluid passing through the pipe is electrically conductive. As a result of the conductor movement through the magnetic field, an electric potential or electromotive force (EMF) is induced in the fluid which is detected by the electrodes. Operation is thus based on Faraday's law of electromagnetic induction.
Typically, electronic circuitry in the magnetic flowmeter is designed to be used with different types of flowtubes. It is also desirable to interchange different types of flowtubes with the same flowmeter circuitry. The coils in the flowtube are designed for different drive currents. This information is specified by the manufacturer. A larger drive current provides a signal of greater magnitude. However, improperly large drive currents can heat the coil, causing damage. Further, improperly large currents do not allow the induced magnetic field to settle, leading to improper measurement. Thus, the flowmeter circuitry must be configured by an operator to provide the proper drive current to the flowtube. Configuring of the electronic circuitry is time consuming and introduces an additional step that must be performed during flowmeter installation or reconfiguration. Additionally, the step of configuration can be a source of operator error. An error in selecting the drive current could damage the flowtube resulting in needless expense or could be a source of errors in flow measurements.